In the exploration of oil in offshore, deep sea locations, it is necessary that the oil drilling equipment be supported on a platform at some distance above the ocean floor. For water depths greater than about 1000 ft. the weight and foundation requirements of traditional offshore structures make them less attractive than other design forms. Two such traditional forms are the guyed tower and tension-leg platform.
The guyed-tower concept is illustrated in FIG. 2. It consists of a uniform cross-sectional support structure held upright by several guy lines that run to clump weights on the ocean floor. From the clump weights, the lines then run to conventional anchors to form a dual stiffness mooring system. Under normal operating loads, the clump weights remain on the sea floor and lateral motion of the structure is restrained. However, during a severe storm, the clump weights are lifted off the sea floor by loads transferred from the structure to the clump weights through the guy lines. This action permits the tower to absorb the environmental loadings on it by swaying back and forth without overloading the guy lines. The guyed-tower concept is presently considered to be applicable to water depths of about 2000 ft.
FIG. 3 illustrates the tension-leg concept. In this design, vertical members are used to anchor the platform to the sea floor. This upper part of the structure is designed with a large amount of excessive buoyancy so as to keep the vertical members in tension. Because of this tension, the platform remains virtually horizontal under wave action. Lateral excursions are also limited by the vertical members, since such movements necessarily cause them to develop a restoring force. A major advantage of the tension-leg concept is its relative cost insensitivity to increased water depths. At the present time, it appears that the main limitation on the tension-leg platform arises from dynamic inertia forces associated with the lateral oscillations of the platform in waves. These become significant at water depths of about 3000 ft.
In the use of either of these prior art designs, substantial underwater welding in physically restrictive environments is required. Such costs are significantly increased with the increase in depths in which the structures are used. Also, the depths at which these platforms may be used are limited because of costs and stability problems.
Because of the substantial costs of using either guyed tower platforms or tension leg platforms in substantial depths, drill ships have been used. However, availability of such ships as well as maintaining such ships in position over the well site require expensive precision navigational equipment. Such equipment adds significantly to the costs involved and often renders this option unacceptable.
The inventor intends to rectify the above shortcomings by usage of manufacturing and prefabricating facilities where computer controlled precision equipments are available. With good quality control on both materials and equipments, high quality modular space components can be casted, blow-molded, compressed and prefabricated. Such modular components shall contribute to substantial reduction in the need for the expensive underwater welding work, which is plagued with crack and fatigue failure problems.
The dodecahedrous float platform (DFP) introduced here is a hybrid of the guyed tower and the tension leg platform systems, using equi-angular rigid Y modular space components. The shortened erection time, reduction in the need for underwater welding replaced by field bolts simple fabrication and erection procedures should contribute to a substantial cost reduction on offshore platform construction, which currently run at $1.2 billion dollars on the 3000 ft. deep Mars project (p. 8, Oct. 18, 1993--Engineering News-Record, McGraw Hill).